A typical digital halftone color proofer uses four colors. Each of the colors, cyan, magenta, yellow, and black (CMYK) are imaged at constant exposure and 1800 dpi. The donors consisted of a visible dye and an infrared dye, which absorbed laser energy and sublimate, causing an amount of visible dye to transfer to an intermediate receiver sheet. The amount of dye transferred is proportional to the amount of laser energy deposited onto the donor. The resultant density level of a solid image resulting on the print is determined by the exposure delivered to each donor. Laser power levels are used at two drum speeds to adjust the exposure for each colorant.
The digital color halftone proofer may have a writing resolution of 2540 dpi or 2400 dpi. Color proofers may use both laser power and drum speed to adjust the exposure for each colorant. A drum speed increment of 25 RPM allows running close to the maximum laser power most of the time thereby increasing print throughput. Typically, color proofers image one bitmap at one exposure per pass. The high writing resolution and the small spot size, approximately 25 um, are used to simulate center weighted halftone dots and text that are normally imaged on a printing press.
Previously, the need to create a multilevel printer with resolutions greater than 1200 dpi did not exist since writing at 1200 dpi produces good looking binary text. The human eye can distinguish image features up to approximately 100 cycles per inch. The unaided eye may not distinguish frequencies greater than 100 cycles per inch, (4 cycles per mm). The maximum print density on paper is approximately 2.0 Status T density. The eye may see a 0.01 density change as a just noticeable difference. A printer with a dynamic range of 200 levels, a maximum Status T density of 2.0, and a writing resolution of 100 cycles per inch, produces a level of quality such that one level change on one pixel produces a visible difference.
A center weighted halftone print produces tone scale by changing the size of the halftone dot. Halftone screens have a screen ruling or halftone dot spatial frequency, and a screen angle. Higher spatial frequencies are used to eliminate the visibility of the halftone. A 150 dpi halftone screen is above the 100 cycles per inch human eye threshold and is not visible to the naked eye.
For a Status T solid density of 2.0, paper Status T density of 0.10, and a 50% dot; a 0.01 density change is a 1.17% change in dot area requiring a minimum writing resolution of 1387 dpi to simulate a halftone dot image. At the 90% dot level an 0.01 density change is a 0.26% change in dot area requiring almost 3000 dpi writing resolution. Normally for imaging halftone screens at 1800 to 2540 dpi writing resolution 0.01 density changes can be achieved without using multiple exposure levels. Prior to the present invention, no one has produced a printer capable of writing multiple exposures at these resolutions because such a printer would exceed the capability of the unaided human eye, making it difficult for an observer to differentiate its benefits. This would make the printer more expensive and less competitive against lower cost products.
Color proofers create halftone bitmaps of cyan, magenta, yellow, and black color planes using a raster image processor (RIP). Customer artwork is composed into pages using software such as Quark Express™ or Adobe InDesign™. These pages may consist of color images, black and white images, artwork, linework, and text. Images may be continuous tone, multilevel, or binary. The pages may also contain PDF or PostScript codes. The RIP processes the input pages and creates halftone bitmap files for each color plane at the writing resolution of the printer. The RIP converts multilevel input, such as the pixels in a continues tone image, into halftone dots of the appropriate size.
To calibrate the halftone dot image, a dot gain correction curve is added to the continuous tone image prior to raster image processing the continuous tone image into a halftone bitmap. This imposes the dot gain correction onto the rendered halftone dot so that the output print measures the correct density and visibly matches the printed sheet. The calibration curve can be created by known methods, such as those described in U.S. Pat. Nos. 5,255,085 and 5,293,539. Percent dot area is calculated using the Murray-Davies equation from measured densities. The Murray-Davies equation is defined in ANSI/CGATS, 4-1993, 1993, p. 7. This calibration method adjusts the image tone scale or dot gain by changing the size of the simulated halftone dot.
Another method of calibration is to filter the bitmap image to change the size of the simulated halftone dot. U.S. Pat. No. 5,250,934 discloses a method of shifting and adding a bitmap image with itself to thin the image displayed. U.S. Pat. No. 5,250,934 discloses a method of setting a bit to an intermediate level if it is diagonally between two active bits using shifting, logical AND, and a logical OR operation.
In correcting for the tone scale of the image using the previous techniques the size of the written halftone dot is changed in the bitmap image to generate a print with the correct measured density. A method of correcting the tone-scale or dot-gain of the image without changing the size of the halftone dot would generate a proof that more closely matches the press sheet. Instead of changing the size of the halftone dot, a bitmap consisting of the edges of the halftone dots for each bitmap plane is created. The original bitmap is printed at a first exposure to obtain the nominal solid density required. The additional bitmap can be imaged at a second exposure to create additional density for each halftone dot and change the dot gain in the proof without changing the size of the written halftone dots. This method requires making an additional exposure pass which increases the time required to create the proof.
Some printers also have a feature called recipe color which allows the customer to image a bitmap with more than one colorant and exposure. This allows the customer to mix his own color and simulate printing with a unique spot color ink. For instance, the customer may desire to print using a custom red ink which is simulated by imaging a magenta donor at a first exposure to achieve 1.0 magenta density followed by imaging yellow donor at a second exposure to achieve 0.50 yellow density. By imaging the 1.0 magenta and 0.50 yellow passes with the same bitmap in registration on the proof a solid red color is created. This technique also requires additional exposure passes to image each special or spot color desired in the job. Recipe colors may be created using combinations of any of the donors available in the printer. The additional exposure passes increase the printing time required resulting in lower print throughput.
A problem can result from incorrect color of the overprints between cyan, magenta, yellow, and black. When imaging the primary color planes the exposure is set to achieve the density of the solid primaries. This results in slight color errors when the primaries overlap each other when compared to the inks used in the printing process that must be matched. For example, the overprint of cyan and magenta yields the color blue. However the blue from the printer may not exactly match the blue achieved using inks on press. A blue bitmap can be created by the logical AND of each bit in the cyan bitmap with each bit in the magenta bitmap. Then the blue bitmap is imaged as a recipe color. However this technique also requires additional exposure passes, which decreases the throughput of the printer.
Current printers have a set of laser diode controllers for each laser channel as described in U.S. Pat. No. 5,966,394. The drum speed and translation speed have to remain constant within each imaging pass. To image multiple levels within a pass the laser power needs to be modified. A need has existed to keep the existing laser diode controller for all of its benefits described by U.S. Pat. No. 5,966,394, yet replace the data path driving the laser diodes and upgrade existing equipment.